The enkephalin polypeptides, methionine-enkephalin and leucine-enkephalin are well-known analgesics endogenous to brain tissue, originally identified and isolated by Hughes (Brian Research, 88:295-308, 1975 and Nature, 258:577-579, 1975). Numerous analogues and derivatives of the enkephalins have since been developed which variously act as agonists or antagonists of endogenous opiate peptides by binding to opiate receptors of neural tissues. The prior art enkephalin modifications, however, have not exhibited marked specificity for binding sites. Further, prior art enkephalin analogues have frequently lacked sufficient affinity for the receptor molecules, and are susceptible to enzymatic degradation.